The micro-organisms of the soil

by Edward J. Russell; Rothamsted Experimental Station

At first sight it might be thought comparatively easy to settle a question of this kind by examining soil under a microscope or by sterilising it and introducing successively bacteria and known types of protozoa. Unfortunately neither method is simple in practice. It is impossible to look into the soil with a microscope, and methods of teasing-out small pieces of soil on a slide under the high, or even the low power, give no information, because the particles of soil have the remarkable power of attracting and firmly retaining protozoa, and no doubt bacteria as well; indeed, for protozoa (which have been the more fully investigated) there seems to be something not unlike a saturation capacity (see [Fig. 9], p. 78). Further, complete sterilisation of soil cannot be effected without at the same time altering its chemical and physical properties, and changing it as a habitat for micro-organisms. Cutler has, however, overcome the difficulties and shown that the introduction of protozoa into soils sterilised and then reinfected with bacteria considerably reduces the numbers of these organisms.

The method adopted, therefore, is to take a census of population and of production. Counting methods are elaborated, and estimates as accurate as possible are made of the numbers of the various organisms in a natural field soil at stated intervals. Simultaneously, wherever possible some measure is taken of the work done. The details of the census are finally arranged in consultation with the Statistical Department, to ensure that the data shall possess adequate statistical value. From the results it is possible to adduce information of great value as to the life of the population, the influence of external conditions, etc.

The most important investigation of this kind carried out at Rothamsted was organised by Mr. Cutler.[3] A team of six workers was assembled, and for 365 days without a break they counted every day the ciliates, the amœbæ, the flagellates, and the bacteria in a plot of arable ground, distinguishing no less than seventeen different kinds of protozoa. The conclusions arrived at were carefully tested by the Statistical Department.

Of the protozoa the flagellates were found to be the most numerous, the amœbæ came next, and the ciliates were by far the fewest. The numbers of each organism varied from day to day in a way that showed conclusively the essentially trophic nature of the protozoan population. The numbers of amœbæ—especially Dimastigamœba and of a species called α—were sharply related to the numbers of bacteria: when the amœbae were numerous the bacteria were few, and vice versa. Detailed examination showed that the amœbæ were probably the cause of the fluctuations in the bacterial numbers, but Mr. Cutler has not yet been able to find why the amœbæ fluctuated; it does not appear that temperature, moisture content, air supply or food supply were determining causes. The flagellates and ciliates also showed large fluctuations, amounting in one case—Oicomonas—to a definite periodicity, apparently, however, not related to bacterial numbers, or, so far as can be seen, to external conditions of moisture, temperature and food supply, and showing no agreement with the fluctuations of the amœbæ. However, one cannot be certain that lack of agreement between curves expressing protozoan numbers and physical factors implies absence of causal relationships: the observations (though the best that can yet be made) are admittedly not complete. If we saw only the end of the bough of a tree, and could see no connection with a trunk, we might have much difficulty in finding relationships between its motion and the wind; whatever the direction of the wind it would move backwards and forwards in much the same way, and even when the wind was blowing along the plane of its motion it would just as often move against the wind as with it.

Meanwhile evidence was obtained that the twenty-four hour interval adopted by the protozoological staff was too long for bacteria, and accordingly the Bacteriological Department, under Mr. Thornton, refined the method still further. Bacterial counts were made every two hours, day and night, for several periods of sixty or eighty hours without a break. The shape of the curve suggests that two hours is probably close enough, and for the present counts at shorter intervals are not contemplated. But there is at least one maximum and one minimum in the day, although the bacterial day does not apparently correspond with ours, nor can any relationship be traced with the diurnal temperature curve.

The nitrate content of the soil was simultaneously determined by Mr. Page and found to vary from hour to hour, but the variations did not sharply correspond with the bacterial numbers; this, however, would not necessarily be expected. The production of nitrate involves various stages, and any lag would throw the nitrate and bacterial curves out of agreement. There is a suggestion of a lag, but more counts are necessary before it can be regarded as established.

Examination of these and other nitrate curves obtained at Rothamsted has brought out another remarkable phenomenon. No crop is growing on these plots, and no rain fell during the eighty hours, yet nitrate is disappearing for a considerable part of the time. Where is it going to? At present the simplest explanation seems to be that it is taken up by micro-organisms. A similar conclusion had to be drawn from a study of the nitrogen exhaustion of the soil. The whole of the nitrate theoretically obtainable from the organic matter of the soil is not obtained in the course of hours or even days; in one of our experiments at Rothamsted nitrification is still going on, and is far from complete, even after a lapse of fifty-three years. The explanation at present offered is that part of the nitrate is constantly being absorbed by micro-organisms and regenerated later on.

Now what organisms could be supposed to absorb nitrates from the soil? Certain bacteria and fungi are known to utilise nitrates, and one naturally thinks of algæ as possible agents also. Dr. Muriel Bristol was therefore invited to study the algæ of the soil. Her account is given in [Chapter VI.] She has found them not only on the surface, but scattered throughout the body of the soil, even in the darkness of 4 inches, 5 inches, or 6 inches depth, where no light can ever penetrate, and where photosynthesis as we understand it could not possibly take place. Some modification in their mode of life is clearly necessary, and it may well happen that they are living saprophytically. Dr. Bristol has not yet, however, been able to count the algæ in the soil with any certainty, although she has made some estimates of the numbers.

The quantitative work on the soil population indicates other possibilities which are being investigated. There is not only a daily fluctuation in the numbers, but so far as measurements have gone, a seasonal one also. There seems to be some considerable uplift in numbers of bacteria, protozoa, and possibly algæ and fungi in the spring-time, followed by a fall in summer, a rise in autumn, and a fall again in winter. At present we are unable to account for the phenomenon, nor can we be sure that it is general until many more data are accumulated.

In the cases of the protozoa and the algæ, there was a definite reason for seeking them in the soil.